FIG. 1 illustrates a classic prior art feedback control system for controlling position. In FIG. 1, the system receives a position command 100. The command position is subtracted from the actual position by adder 102, generating a position error signal. The error signal is filtered by a compensation filter 104. The compensated error signal is received by a motor driver 106, which in turn drives a motor 108, which in turn causes some object to move. The actual position of the object being moved is measured by a position encoder 110, and the measured position is fed to adder 102.
Position encoders, as depicted in FIG. 1, 110, are used to generate an electronic signal that indicates an absolute mechanical position, or an incremental mechanical movement relative to a reference position. Rotary encoders and linear encoders are known. There are many known ways of generating a position signal, including magnetic sensors, capacitive sensors, and optical sensors. In a typical optical position encoder, a wheel or strip, having a scale or light mask comprising a series of rectangular openings with opaque areas between the openings, is mounted between a light source and a photodetector. As the wheel or strip is moved relative to the photodetector (or vice versa), the rectangular openings, and opaque areas between the openings, alternately transmit or block light to the photodetector. It is common to provide two scales, and two detectors, with the openings of one scale offset by half the width of an opening. The encoder then generates two binary quadrature signals. There are commercially available integrated circuits for generating position and direction signals from quadrature signals.
Depending on the relative sizes of the photodetectors and the openings in the scale, the signal from the photodetectors may be triangular or may be approximately sinusoidal. Typically, a triangular or sinusoidal signal is compared to a threshold by a comparitor circuit to generate a binary quadrature signal. For a binary quadrature signal, the resolution is typically one fourth of the pitch of the openings in the scale. Higher resolution may be obtained by measuring intermediate points along a triangular or sinusoidal signal. However, some techniques measuring intermediate points are sensitive to the amplitude of the sensor signal, and discontinuities may occur at encoder signal peaks.
One system has been described in which two sinusoidal signals are obtained and an analog error signal is generated without having to compute Arc-Sin and Arc-Cosine functions. In addition, the accuracy of the error signal is insensitive to sensor signal amplitude, and there are no discontinuities. See, William C. Gibson et al., "POINTING AND GUIDANCE OF THE BUSS TELESCOPE," Proceedings of the Society of Photo-optical Instrumentation Engineers, Volume 28, pp. 249-260, March 1972. The Buss telescope position control system generated position feedback signals using magnetometers sensing the earth's magnetic field. The control system used multiplying digital-to-analog converters, and operational amplifiers, to generate an analog error signal without having to compute Arc-Sin and Arc-Cosine functions.
A surveying instrument has been described, in which an encoder generates a single sinusoidal signal. However, the encoder is not used within a closed-loop control system. See, Alfred F. Gort, "THE HEWLETT-PACKARD 3820A ELECTRONIC TOTAL STATION," Hewlett-Packard product marketing literature, document number 5952-9136, published approximately 1978. In the angle encoder for the 3820A, a glass disc had a transparent area bounded by opaque metal film patterns, where the width of the transparent area varied sinusoidally. Photo-diodes, each wider than the widest part of the transparent area, generated an incremental sinusoidal angle position signal.
In systems where space is confined, or in systems in which the size of the photo-sensitive elements may be determined by factors unrelated to position sensing, an encoder as described for the 3820A surveying instrument may not be suitable because of the required width of the sinusoidal pattern. For example, an optical image scanner has been proposed in which a position encoding strip is sensed by elements of an array of photosensors. The array of photosensors is primarily used for image sensing, but some of the photosensors are dedicated to position sensing. An individual photosensor in an array of photosensors for a document scanner may, for example, sense an area having a width of 0.00167 inches (0.042 mm). If an encoding pattern is used as described for the 3820 surveying instrument, the pattern would have to be only a few tens of micrometers wide, or alternatively, many photosensor elements would have to be dedicated to sensing position. There is a need for a feedback system having the advantages of the general type of control system described for the Buss telescope, with precision sinusoidal incremental position encoding as described for the 3820 surveying system, but enabling use of small photosensor elements, and without requiring dedication of a large quantity of photosensor elements for position sensing.